Caption limit for Media Tour

Is there a size limit to the Caption text box for a Media Tour screen? And/or does the size of the image influence the space alotted for the Caption text box?  I am want to use this feature to create a series of images with explanations.

Also, does anyone know if the images within a series of Media Tour screens packaged together need to be the same size. 

Thanks for any advice you have! Cheryl

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